This invention relates to a hot stamping process and apparatus for forming sheet metal alloys with low formability at room temperature. In particular, this invention relates to a warm/hot sheet forming operation with rapid pre-heating process on the press table by direct electric current application to the workpiece using two electrode sets contacting at two opposite edges of the workpiece.
In the plastic forming processes of various metal parts, heating is sometimes necessary before a forming operation. In the metal deformation processes such as forging, extrusion, rolling etc. the workpiece is heated above its recrystallization temperature prior to subsequent forming operation and these processes are generally referred as hot working.
In common hot forming techniques, the metal workpiece is heated in a fuel-fired or electric furnace before mechanical forming operation performed by forging, rolling, extrusion, drawing etc. During the period in which the workpiece is removed from the furnace and placed on the press table between the dies, a considerable amount of heat is lost from the workpiece. Generally, heat loss is proportional to surface area of the workplece. Heat is held by the mass of the original workpiece and heat loss occurs in peripheral area of the workpiece by radiant, convective and conductive means. Increase in peripheral area/mass ratio of the metal workpieces results in more rapid cooling phenomena during handling from furnace to forming machi ne, and thus, hot forming of such thin metal workpieces become difficult or practically impossible in some cases. Radiant heat loss becomes dominant at high temperatures, because it is proportional to fourth power of the workpiece temperature, and while conductive heat loss is linearly proportional to temperature of the workpiece.
Hot forging including preheating at a furnace, handling to forming machine and then compression forming is a widely used hot working process for a long time all around the world. In such a process heat loss of the hot bulk workpiece can be kept in an acceptable level and does not prevent the operation.
In hot working of a sheet metal workpiece with thickness between 0.6 and 3 mm such as articles used in automotive industry, peripheral area/mass ratio is too much and such a workpiece cannot keep its temperature sufficiently during handling of hot blank to be placed between dies after furnace heating. A hot blank sheet loses a considerable amount of heat and its temperature rapidly decreases below hot working temperature range within a few seconds. Most of high strength alloy steels, aluminum and magnesium alloys are temperature sensitive and they are only formable within narrow ranges of temperature. Such a heat loss becomes particularly severe for high strength and thin alloy sheets, and thus, subsequent hot working becomes practically impossible. Therefore, there is not a widely used hot stamping method in use for production articles made from high strength alloy sheet for particularly automotive industry.
In practice, such a thin sheet can keep its temperature only a few seconds for subsequent forming process. For example, in a steel blank sheet in 1100xc2x0 C. temperature with 1 mm thickness, temperature decrease rate is more than 100xc2x0 C./sec. Heating the workpiece to higher temperatures is not a solution, because radiant heat loss varies with fourth power of the temperature and temperature fall becomes more severe. On the other hand, overheating may alter microstructure (grain size, structure, elongation rate, formability, strength etc.) of the workpiece or cause surface oxidation.
Although there are much higher strength steel and aluminum alloys, currently used stamping technology can not form such metal thin sheets by currently used sheet stamping technology due to lack of formability. Thin sheets made of such alloys can offer very higher strength up to three or four times more strength than of currently used sheets in automotive production. Such metal blanks principally can only have adequate formability in high temperature rates and within tight range.
The most important utilization area of the invention is the automotive industry. One of the main challenges for the today""s automotive industry is xe2x80x9cHow to produce lightweight and stiff auto chassis and body construction in mass production with high quality low costxe2x80x9d. Stamped sheet articles consist of (app. % 50 -60) most of auto body weight. There are many weight loss programs carried out by car companies, suppliers etc all around the world in efforts to make new production technologies more responsive to needs of the low fuel consuming vehicles of tomorrow. There are many technical teams in the automotive world, in collaboration with the national labs, universities and suppliers, are working to reduce vehicle weight as compared to today""s compact and midsize family sedans. Therefore, there is a widespread tendency to use widespread tendency to use relatively higher strength steels and light aluminum and magnesium alloys in the automotive industry.
From aspect of safety, energy rate that can be absorbed elastically during a crash by a metal auto part until plastic deformation limit is proportional to second force of its yield strength. However, a single part made of relatively higher strength metal might require more stamping stages than a comparable part or the part may have to divide into two or more pieces that are then joined together. Nevertheless, these solutions add time and cost to the manufacturing process. Thus, engineers have been trying to find other methods to replace or complement the conventional mechanical stamping process in order to fully realize the potential weight savings of using of higher strength steel and aluminum components. On the other hand, such materials cause more wrinkles and tears during manufacture and require significant try-out modification and completion works for dies and tools requiring higher cost, time and labor.
The main principle of the invention is to achieve both direct heating of the blank by current application on the press table and hot stamping operation performed as subsequent process achieved in one place (press table) without requiring any handling operation of the workpiece resulting severe temperature fall preventing such an hot shaping process. Temperature fall at the hot thin sheet during handling from pre-heating furnace to press table is so severe that it is practically impossible to keep its heat sufficiently until end of the stamping process between two dies.
For example, steel sheet thickness, 1 mm , T=1100xc2x0 C., Temperature decrease rate =110xc2x0 C./sec Heat Energy Equation of this process:                                           R            ⁡                          (                              1                +                                                      α                                        *                                                                *                                    ⁢                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  T                                            )                                ⁢                      I            2                                     Heat rate generated
inside the Workpiece
         by Current                    =                                    [                          m              ⁢                              c                                *                                                    *                            ⁢                                                ⅆ                  T                                /                                  ⅆ                  t                                                      ]                                Heat rate held mass
   of the workpiece                          +                              {                          2              ⁢                              A                                *                                                    *                            ⁢              4.96              ⁢                                                k                                    *                                                          *                                [                                                      (                                                                  T                        s                                            /                      100                                        )                                    4                                                                          Heat loss by
   radiation                          -                              (                                          T                e                            /              100                        )                    4                      }    +            A      ⁢              β                *                            *            ⁢      Δ      ⁢              xe2x80x83            ⁢      T              Heat loss by
 Convection      
where R electric resistance of the workpiece (Cold), xcex1 Resistance increase coefficient by temperature, xcex94T temperature increase of the sheet, I current, m mass of the workpiece, C Specific thermal capacitance, A Area (one side) of the WP, Ts temp of the sheet, Te peripheral temperature, xcex2 Convection coefficient, k radiant heat transfer coefficient.
The process ensures instant temperature rate of the hot sheet at the stamping moment by controlling principal parameters of the process such as, current, current application time, stamping time etc within one machine. This process can be applied in mass production of articles made from high strength alloy sheets for automotive industry, because whole process including, heating, stamping, cooling within dies is performed in one machine within a few seconds. It""s very important to prevent thermal or mechanic distortions of formed article during cooling after hot stamping. Cooling must be performed without any distortion and preferably; formed sheet should be removed from the dies after sufficient temperature fall. Dimensional stability and sufficient strength (after cooling) should be ensured during removing of the stamped part. Particularly, automotive industry demands strict dimensional tolerances. This process achieves instant cooling of the workpiece without distortion by means of cooling under pressure of cold dies. The dies are kept within a previously determined temperature range that is fairly lower than forming temperature of the workpiece. A little amount of heat is gained by dies by contact of the hot workpiece at each process cycle. On the other hand, the dies continuously lose heat because their temperature will be slightly higher than room temperature during mass production.
The process starts with current application to the workpiece for a few seconds and temperature of the blank sheet is reached previously determined rate to provide sufficient formability characteristics in the workpiece such as elongation rate, yield strength etc. Until this certain temperature rate is provided in the workpiece, the dies are not in contact with hot workpiece. At least one die is moved toward the hot sheet and sheet is stamped. Temperature of the dies is fairly lower than hot forming temperature and slightly higher than room temnperature. An instant cooling process is achieved at the end of the stamping while the sheet is being completely compressed with two dies and it is very important to prevent thermal or mechanical distortions in order to provide strict dimensional tolerances.
Similar heating process is also needed in sheet bending and prototype production processes if the blank sheet is made from high strength alloy metal. In bending process of such blanks, similar heating operation is applied before bending. In prototype production with use of one die, the main principle of the invention is applied and these processes are explained below.